There are two types of rotary-electric machines. The first type is a radial-gap rotary-electric machine in which the gap between the rotor and the stator extends in the radial direction of the shaft. The second type is an axial-gap rotary-electric machine in which the gap between the rotor and the stator extends in the axial direction of the shaft.
A radial-gap rotary-electric machine of a very high speed of 10000 rmp or more generates a considerably large centrifugal force while its shaft is rotating. In a high-speed radial-gap rotary-electric machine having permanent magnets incorporated in the field system of the rotor, a nonmagnetic holding ring having a considerably large thickness is wound around the outer periphery of the permanent magnets so that the magnets may not disintegrate and moved from the rotor. In a high-speed radial-gap rotary-electric machine having a coil used in the field system of the rotor, a holding ring holds the end ring of the coil.
An axial-gap rotary-electric machine comprises a rotor disc, an armature coil and a stator disc which are arranged along the axis of the shaft, opposing one another with gaps among them. A coil or permanent magnets are mounted on the rotor disk, forming poles of a magnetic field.
When the conventional radial-gap rotary-electric machine rotates at a very high speed of 10000 rmp or more, its centrifugal force becomes considerably great, and the rotor coil may fail to withstand the force and may thus be broken.
In a radial-gap rotary-electric machine which has, as shown in FIG. 1, permanent magnets 101 in the field system, a nonmagnetic holding ring 102 is required which is thick enough to prevent the magnets 101 from being moved from the rotor. Being made of nonmagnetic material so that a magnetic circuit may not be short-circuited, the holding ring 102 inevitably has a long magnetic gap. The electromotive force is consumed in the gap in a large amount, reducing the output of the rotary-electric machine. In FIG. 1, numerals 103, 104, 105, 106, 107, 108, and 109 designate the stator frame, the stator core, the coil, the rotor, the rotor yoke, the shaft, and the gap, respectively.
In an axial-gap rotary-electric machine schematically shown in FIG. 2, the disc-shaped rotor yoke 110 is made of magnetic metal such as soft iron. An axial-gap rotary-electric machine of this type therefore has a larger rotor inertia than an ordinary axial-gap rotary-electric machine. It takes this machine a long time to reach a target speed after it has been energized started, and to stop after it has been de-energized. Axial-gap rotary-electric machines of this type are, therefore, considered unsuitable for use in robots or automatic machines which need to accelerated and decelerated quickly. In FIG. 2, numerals 111, 112, 113, 114, 115, and 116 denote the motor frame, the stator yoke, the coil, the bearing, the shaft, and the permanent magnets, respectively. The coil 113 has an U phase 113a, a V phase 113b, and a W phase 113c.
In the conventional axial-gap rotary-electric machine, the rotor cannot have a plurality of discs so that the machine may have a large capacity and rotate at high speed. This is because stator discs each having an armature coil and rotor discs need to be alternately arranged in the axial direction of the shaft. Since the capacity is increased by using only one rotor disc, the rotor must be one having a large diameter, making it difficult for the machine to rotate at high speed or to generate a large output.
In the axial-gap rotary-electric machine of this type, the yoke 110 of the rotor defining a magnetic flux passage is made of magnetic material such as soft iron. The rotor of the axial-gap rotary-electric machine is inevitably heavy and may have a problem in terms of mechanical strength against a centrifugal force. Furthermore, the load on the bearing increases in the axial-gap rotary-electric machine of this type, the critical speed, i.e., the maximum allowable speed, for the shaft decreases. The shaft cannot rotate at high speed.
An object of the present invention is to provide an axial-gap rotary-electric machine which can rotate at high speed.
Another object of the invention is to provide an axial-gap rotary-electric machine which can generate a large output.
Still another object of this invention is to provide an axial-gap rotary-electric machine which can rotate at high speed and generate a large output.